The present invention relates to a process for producing monovinylidene aromatic polymers.
High molecular weight vinyl aromatic polymers, particularly polymers having weight average molecular weights (Mw) of greater than 300,000, have been typically produced by anionic polymerization rather than by free radical polymerization due to the slow polymerization rates used in free radical techniques to achieve high molecular weight polymers. However, anionic polymerization processes require expensive anionic initiators, such as organolithium compounds, and tend to produce discolored products due to the presence of residual lithium-containing salts.
High molecular weight vinyl aromatic polymers have also been produced by free radical polymerization in the presence of a soluble organic acid having pKa of 0.5 to 2.5, as in U.S. Pat. No. 5,145,924. However, in this process the acid does not bind to the polymer and can migrate from the polymer during use, which can cause corrosion of mold surfaces.
Therefore, it remains highly desirable to provide an improved process to produce high molecular weight monovinylidene aromatic polymers, without the disadvantages of the prior art.